New electrophysiological methods are being utilized to study the development of excitability and synaptogenesis of mammalian CNS neurons in vitro. Developing neurons, previously inaccessible to physiological investigation, have been examined using a variety of patch electrode techniques. Developmental changes in the action potential mechanism during the development of embryonic mouse spinal cord neurons in vitro have been found to be related to changes in the number and/or distribution of sodium channels rather than to changes in the kinetics of the sodium current. The appearance of excitatory amino acid receptors precedes the onset of excitatory synaptic activity, but the sensitivity to excitatory amino acids increase rapidly during synaptogenesis in culture. Different types of excitatory amino acid receptors exhibit differential developmental regulation.